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Patented Dec. 2, 1952 PREPARATION OF 3,7-DIAMINODIBENZO- THIOPHENEDIOXIDE 2,8 DISULFONIC ACID Sien M Tsang, Bound Brook, N. J assignor to American Cyanamid Company, New York, N. Y., a corporation of Maine No Brawing. Application September 13, 1950, Serial. No. 184,707

Claims. 1

The present invention relates to the preparation of diaminodisulfonic acids of the dibenzothiophenedioxide series. More particularly, it is concerned with the sulionation of compounds such as benzidine and 3,7-diaminodibenzothiophenedioxide. Still more specifically, it is concerned with the problem of increasing the yield of the desired disulfonic acids and the minimizing of the production of isomeric and polysulionic acids.

Diaminodisulionic acids of the dibenzo-thiophenedioxide series are important intermediates for dyestuffs and the preparation of fluorescent brighteners for cellulosic and other materials. Despite their importance, however, available methods for their preparation have been found sufiiciently unsatisfactory as to limit the production and use of these materials. For example, one of the most important of these compounds is 3,7-diaminodibenzothiophenedioxide-2,8-disulionic acid of the formula nsmsou1 H2N- \S/ NH2 This compound is usually prepared by sulfonating benzidine or 3,7-diaminodibenzothiophenedioxide. However, sulfonation reactions, in addition to the desired product, yield various byproducts consisting largely of isomers and polysulfonic acids. For the most part, these by-products are inferior to the 3,7-diaminodibenzothio phenedioxide-Z,S-disulionic acid, with respect to substantivity, fluorescence and other desirable properties. Formation of these by-products lowers the yield and purity of the desired product. Various attemps have been made to minimize byproduct formation, ior example, by the use of milder sulfonating conditions, but this results only in incomplete sulionation.

Despite the previous lack of success with such reactions, the products are highly desirable. If they can be produced in increased purity, or at decreased expense, there remains a demand for the products industrial development. It is, therefore, the principal object of the present invention to devise a process whereby the sulfonation reaction may be successfully carried out with r a minimum production of by-products.

Despite the previously-known difiiculties, the solution to the problem presented in making the present invention has been found to be quite simple. It has been found that by heating the sulfon'ation reaction, product for a sufficient length of time, in sulfuric acid of concentrations lower than those used for sulfonation, many of the by-products are converted to the same isomer. As a result, it is easily possible to markedly increase the yield of the desired disulfonic acid, for example, the 2,8-disulfonic acid noted above, over the yield obtainable without using the aftertreatment.

It is rather surprising that this result can be obtained in such a simple manner. Particularly is this true in view of past experience. In previous attempts to increase the yield by minimizing by-product formation, as for example by using milder reaction conditions, the only change in result was to obtain incomplete sulfonation. In addition, it is wholly unpredictable that in the present series of compounds, the less valuable sulfonic acids should be selectively converted into precisely the particular isomer which is the most important member of; the series.

It is an advantage of the present invention that the extra processing is extremely simple to carry ut. It is possible to first isolate the 2,8- disulfonic acid, formed in the conventional sulfonation process, by any known means, for example as a. monoamine salt of a tertiary base, as described in our co -pending application Serial No. 168,379, filed June 15, 1950. This is then followed by an application of the acid treatment of the present invention to the other isomeric and polysulfonic acids remaining. In this way, valueless material is converted into a valuable product. An even simpler mode of operation is to carry out the sulfonating step, then, without isolating any product, dilute the reaction mixture and carry out the heating for a sufiicient time to efiect the conversion; thus very appreciably increasing the yield of desired product isolated.

While the precise limits of time, temperature and concentration required for the conversion are not particularly critical, there is, of course, an inter-relationship. For example, if the strength of acid is lowered, for example to about 10%, then the time of treatment must be lengthened to hours and more, and the temperature raised above the normal boiling point by the use of superatmospheric pressure. On the other hand, if the acid strength is raised to or even to 101% (weak oleum), then the temperature may be as low as 100 C. and the time as short as 1 or 2 hours. Because of these requirements of time and temperature, while suliuric acid co c ntratio o i i- 0 ar e abl e consider concentrations below about 45% as less practical. Further, it is well known that acid concentrations of about 80% permit the use of ordinary iron equipment and hence offer a technical advantage. Also, at this concentration the increased solvent power of the acid permits the use of larger-sized batches, so that an acid concentration of 80% or higher will, in general, be found most advantageous. Experience has shown that the most satisfactory range is from about 85% to about 100%.

There are also certain practical restrictions on the temperature used. In general, the satisfactory range is found at from about 80-140 C. Below about 80 C., the reaction becomes too slow for all practical purposes. Above about 140 0., additional side reactions occur and become too pronounced. Excellent results are obtained using a treating period of about 2-5 hours, a temperature of about 110-125 C. and an acid concentration of about 85-100%.

The invention is Well suited for use in conjunction with the amine salt method of isolation, described and claimed in the above-noted copending application. The presence of the tertiary amines used in the procedure of that application in no way interferes with the conversion reactions of the present invention. The tertiary amine may be added either before or after the acid treatment of the sulfonation mass, according to the process of this invention; and amine salt of the product crystallizes out and is readily isolated in considerably higher yields than if the treatment of the present invention is omitted.

As a practical procedure in ca-rying out the conversion, it is not necessary to dilute the reaction mixture to the extent that all of the products are completely soluble therein during conversion. At the conclusion of the sulfonating operations, sufficient water, preferably as ice for cooling purposes, is added to effect the desired dilution for the conversion step. During the conversion, some material will be either originally out of solution or will be precipitated during the conversion. This precipitation in no way interferes with completeness of conversion. At the completion of the treatment, the mixture is then further diluted to permit ready isolation of the desired 2,8-disulfonic acid, either as free acid, or as an alkali metal salt, by the addition of sodium or potassium ions, or as a monamine salt of a tertiary base, as described in the aforementioned copending application. These precipitated forms of the desired disulfonic acid are all useful and may be isolated by filtration and washing with acid, brine, or water, respectively.

The practice of the process of the present invention will be more clearly illustrated in conjunction with the following examples. These examples are primarily for illustrative purposes and are not necessarily intended by way of limitation. All parts, except as otherwise noted, are by weight. All temperatures are in degrees centigrade.

Example 1 To 677 parts of 25% oleum there is gradually added with vigorous stirring 84.6 parts of dry finely divided benzidine sulfate. The temperature of the mixture is not allowed to exceed 65 C. After addition is complete, the solution is heated and stired at 67 C. for three hours. This operation produces the intermediate 3,7-diaminodibenzothiophenedioxide.

There is then added 22.5 parts of 85% sulfuric acid and the mixture is rapidly heated to 140 C. and maintained at this temperature for two and heated one hour at 155-160 '4 one-half hours. This operation sulfonates the above-mentioned diaminodibenzothiophenedioxide to a mixture of products, largely the 2,8-disulfonic acid. It is then cooled to about C.

The conversion reaction is carried out by adding about 110' parts of cracked ice to the mixture, thus reducing the sulfuric acid concentration to about stirring the resulting slurry for two hours at 122 C. and drowning into 3000 parts of water.

In order to isolate the desired disulfonic acid, the drowned material is heated to the boil with 33.3 parts of triethylamine. Enough water is then added to dissolve the yellow opaque precipitate. The mixture is cooled slowly to 24 C. and the yellow precipitate filtered and sucked dry. The cake is washed twice with parts of 2% sulfunic acid and dried at 100 C. It represents a 77% yield of the mono triethylamine salt of 3,7 diaminodibenzothiophenedioxide 2,8 disulfonic acid.

If the operation is carried out as described in the first, second and fourth paragraphs of the above example-omitting the conversion step described in the third paragraphthe yield of monotn'ethylamine salt of 3,7-diaminodibenzothiophenedioxide-2,8-disulfonic acid falls to 60% of the theoretical.

Thus, by this simple treatment, a 28% increase in the yield of the desired isomer results.

Example 2 The sulfonation of benzidine sulfate is carried out as described in the preceding example. When sulfonation is completed and the mixture cooled, dilution is carried out by adding 13.1 parts of ice rather than parts as in Example 1. The sulfuric acid concentration resulting is approximately 100%. The mixture is heated for two hours at 122 C. and then worked up in the same manner. An excellent yield of product is obtained.

Similar results are obtained by heating at 80 C., but in this case, a much longer period of time is required, up to 24 hours.

If the conversion reactions, described in this and the other examples, are omitted, but the sulfonation mixture worked up directly, the yields of 3,7-diaminodibenzothiophenedioxide- 2,8-disulfonic acid are much lower.

Example 3 A mixture of 12.3 parts of 3,7-diaminodibenzothiophenedioxide and 55 parts of 20% oleum is C. It is then diluted with 40 parts of 50% sulfuric acid, followed by 20 parts of water, thus reducing the sulfuric acid concentration to approximately 67-68%. Refluxing at 125-130 C. for one hour or more, to carry out the conversion reaction, is followed by the addition of 1000 parts of water and 10.1 parts of triethylamine. The boiling mixture is further diluted, if necessary, to dissolve the solids present and then cooled. The monotriethylamine salt of 3,7-diaminodibenzothiophenedioxide 2,8 disulfonic acid crystallizes from solution in excellent yield.

Example 4 14.1 parts of benzidine sulfate is gradually added to 57 parts of 35% oleum, the temperature being maintained below 70 C. When addition is complete, the mixture is heated at 70 C. for two hours, followed by three hours at 145 C. The concentration of the sulfuric acid is now reduced to 45-50% by the addition of about 57 parts of water and conversion is carried out by stirring and refluxing the resulting slurry at 1'15-120 C. for 4-5 hours. The mixture is then diluted to the volume of 850 parts of water, treated with 10.1 parts of triethylamine and refluxed to dissolve the solids. On cooling, the crystalline triethylamine salt separates in good yield.

If the sulfuric acid concentration is reduced much below the above, substantially longer reflux periods are required to obtain the same results.

Example 5 To 677 parts of 26.8% oleum there is gradually added with stirring 85.1 parts of dry benzidine sulfate of 99.5% purity. During the addition, the temperature of the mixture is not allowed to exceed 65 C. When addition is complete, the mixture is heated to 67 C. for three hours, then treated with 22.5 parts of 85% sulfuric acid, heated rapidly to 140 C., and maintained at this temperature for two and one-half hours. The mixture is then cooled to about 80 C. and treated with 45.5 parts of cracked ice, lowering the sulfuric acid concentration to 95-96%. External cooling is employed to keep the temperature of the mixture below 100 C. When addition is complete, the mixture is heated rapidly to 122 C. and kept at this temperature for two hours.

The mixture is then drowned in about 3000 parts of water and heated to the boil with 33.3 parts of triethylamine. Sufficient water is added to dissolve the yellowish opaque precipitate at the boil. The solution is then cooled to room temperature and filtered. The precipitate of yellow needles is sucked dry, washed with 2% sulfuric acid and dried at IOU-110 C. An excellent yield is obtained.

Example 6 With good stirring, 56.7 parts of benzidine sulfate of purity 99.5% is gradually added to 462 parts of 25% oleum, the temperature being maintained at 65 C. The mixture is held at 67 C. for three hours, then treated with parts of 85% sulfuric acid, rapidly heated to 140 C. and held at this temperature for two and one-half hours. The temperature is adjusted to 80 C. and '11 parts of water is added, reducing the sulfuric acid concentration to approximately 100%. The conversion reaction is carried out by heating at 140 C. for one-half hour and the product worked up in the usual manner. At lower temperatures, longer heating time is required, for example, five hours at 110 C.

Example 7 Benzidine sulfate is sulfonated as described in the preceding example, but the reaction mixture is diluted with only 6.5 parts of water, reducing the sulfuric acid concentration to approximately 101%. The conversion reaction is carried out at 140 C. for one hour, and the product worked up in the usual manner.

Example 8 A portion of benzidine is sulfonated, as for example in the procedure set forth in paragraphs one and two of Example 1 above. To the resultant product, triethylamine is added and the precipitated 2,8-disulfonic acid, in a yield of about 60% of theoretical, is removed.

The residual filtrate contains about 12 parts of a mixture of the valueless isomeric 3,7-diaminodibenzothiophenedioxide sulfonic acids, about 15 parts of triethylamine and the mixture is in about 1000-1200 parts of an 8% aqueous sulfuric acid. This solution is refluxed for about 24 hours and cooled to room temperature. This procedure results in the deposition of about 4.5 parts of the triethylamine salt of the desired 3,7-diaminodi benzothiophenedioxide 2 8 disulfonic acid. This amounts to about 35-40%, based on the mixed sulfonic acids in solution. Further refluxing for about 36 hours results in an additional yield of 3.8 parts, which amounts to about 30% of the theoretical, based on the mixed sulfonic acids in solution. The total conversion is thus about 70% and this figure can be increased by further refluxing.

I claim:

1. In the process of producing 2,8-disulfonic acids of 3,7-diaminodibenzothiophenedioxide in which a member of the group consisting of benzidine and 3,7-diaminodibenzothiophenedioxide is sulfonated with oleum of a strength greater than about 101%, the improved method of increasing the yield of the desired disulfonic acids which comprises heating the reaction product at from about -140 C. in the presence of aqueous sulfuric acid at a concentration above about 10% but less than the concentration used for sulfonation for a sufficient time to substantially increase the 2,8-disu1fonic acid content and isolating the resulting 2,8-disulfonic acid.

2. A process according to claim 1 in which the acid concentration is about 45-101%.

3. A process according to claim 1 in which the acid concentration is about -101%.

4. A process according to claim 1 in which the acid concentration is about 85-101%, the heating temperature is maintained at -125 C. and the reaction time is from 2-5 hours.

5. A process according to claim 1 in which a trialkylamine is added in an amount at least equimolar to the desired 2,8-disulfonic acid and the resultant alkylamine salt is isolated prior to the heating period with the 10-101% acid.

6. The process of producing the 2,8-disulfonic acid of 3,7-diaminodibenzothiophenedioxide from other sulfonic acids thereof, which comprises the steps of admixing said sulfonic acids with sufficient aqueous sulfuric acid of 10-101% sulfuric acid. content to obtain substantial solution at 80-140 C.; heating the mixture at 80-140 C. for suificient time to effect the desired conversion; and isolating the 2,8-disulfonic acid from the resultant 10-101% acid mixture.

7. A process according to claim 6 in which the sulfuric acid concentration is from about 45-101%.

8. A process according to claim 6 in which the sulfuric acid concentration is from about 85-101%.

9. A process according to claim 6 in which the treating temperature is from about -125 C.

10. A process according to claim 6 in which the treatment is carried out in from 2-5 hours at 110-125 C. in sulfuric acid of concentration from about 85-101%.

SIEN MOO TSANG.

REFERENCES CITED The following references are of record in the file of this patent:

Cullinane: Recueil Trav. chim. Pays-Bas, 55, 883-884 (1936). 

1. IN THE PROCESS OF PRODUCING 2,8-DISULFONIC ACIDS OF 3,7-DIAMINODIBENZOTHIOPHENEDIOXIDE IN WHICH A MEMBER OF THE GROUP CONSISTING OF BENZIDINE AND 3,7-DIAMINODIBENZOTHIOPHENEDIOXIDE IS SULFONATED WITH OLEIUM OF A STRENGTH GREATER THAN ABOUT 101%, THE IMPROVED METHOD OF INCREASING THE YIELD OF THE DESIRED DISULFONIC ACIDS WHICH COMPRISES HEATING THE REACTION PRODUCT AT FROM ABOUT 80-140* C. IN THE PRESENCE OF AQUEOUS SULFURIC ACID AT A CONCENTRATION ABOVE ABOUT 10% BUT LESS THAN THE CONCENTRATION USED FOR SULFONATION FOR A SUFFICIENT TIME TO SUBSTANTIAL INCREASE THE 2,8-DISULFONIC ACID CONTENT AND ISOLATING THE RESULTING 2,8-DISULFONIC ACID. 